A simple function generator IS :

- an EPROM with the signal data preprogrammed and the reset point for the counter (D7).
- a binary counter for the signal loop and the frequency.
- a rotary switch for the signal enveloppe selection.
- a NE555 for the counter input (CLK)
- a Digital to Analog convertor.
- some filtering components.

Have fun, low cost developing your signal generator. Use a FLASH instead of an EPROM for development.